Proton-controlled molecular ionic ferroelectrics

Molecular ferroelectric materials consist of organic and inorganic ions held together by hydrogen bonds, electrostatic forces, and van der Waals interactions. However, ionically tailored multifunctionality in molecular ferroelectrics has been a missing component despite of their peculiar stimuli-responsive structure and building blocks. Here we report molecular ionic ferroelectrics exhibiting the coexistence of room-temperature ionic conductivity (6.1 × 10−5 S/cm) and ferroelectricity, which triggers the ionic-coupled ferroelectric properties. Such ionic ferroelectrics with the absorbed water molecules further present the controlled tunability in polarization from 0.68 to 1.39 μC/cm2, thermal conductivity by 13% and electrical resistivity by 86% due to the proton transfer in an ionic lattice under external stimuli. These findings enlighten the development of molecular ionic ferroelectrics towards multifunctionality.

1) The same material ImClO4 were reported in the previous paper of these authors. The reference No. 30 is proton conductivity in the paper of Nat. Commun. 12, 255 4602 (2021). However, the same ImClO4 is also ionic ferroelectrics.
2) What is the physics of ionic ferroelectrics? The author should discuss more about the physics.
3) Figures 1 c and d are not London free energy. It could be Landau free energy or total free energy. Normally, the free energy can be represented by F, and can not use E. In addition, the following related references should be cited to describe the free energy vs polarization in molecular ferroelectrics. Advanced Functional Materials 31 (38), 2104393, ACTA PHYSICA SINICA 69 (21). Figure 3a, the three polarization loops are under different electric filed. However, the saturation polarizations are not equal for different electric fields. 5) Could the author explain the difference of phase transition during cooling and heating?

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Reviewer #2 (Remarks to the Author): Huang et al. reported the molecular ionic ferroelectric ImClO4, which exhibits coexistence of ionic conductivity and ferroelectricity. Interestingly, the coexistence of ionic conductivity and ferroelectricity can trigger the ionic-coupled ferroelectric properties, i.e., ionic controlled ferroelectric behavior. Because ferroelectric-ion coupling is of great significance in the development of the devices with multi-well polarized states and neuromorphic characteristics, the starting point of this work is interesting and worthy of affirmation. However, there are still many problems to be clarified or solved in the manuscript before publication of the work. The main problems are as follows: 1The polarization value of the P-E curve in the manuscript is much lower than that of the same compound reported in the literature, and even lower than that of the same compound reported by the authors themselves (Sci. Adv. 2017, 3, e1701008), why? 2The change of P-E curve with humidity shows that with the increase of humidity, P-E curve appears certain leakage. Is it due to other contributions, rather than an actual increase in polarization? 3Under humidity control, does water adsorb on the surface or enter the lattice? How to exclude the effect of water on simple surface adsorption of the sample? 4Does ImClO4 sample itself absorb moisture? Will the surface of the sample be damaged during this process? 5In what way is the thermal conductivity measured? As we all know, it is difficult to obtain accurate thermal conductivity data, please provide details of this part, especially the thermal conductivity test before and after polarization and at different humidity. 6In Figure 3, the author attributes the dielectric anomaly in a very wide temperature region near 300 K to the mobile proton released from the framework. However, this anomaly also exists in the heating stage of DSC, which is attributed to the structural phase transition. Why? 7In Figure 5b, the P-E loops are enhanced by relatively decreasing resistivity. However, the change is too small to be convincing. 8For Figures S5 and S6, the authors claim that "The repeatable permittivity peak at ferroelectric ordering temperature indicates high crystalline quality of as-grown ImClO4, further evidencing the proton mechanism for the anomaly in permittivity which relates to its cooling history and magnetic field dependence (Figs.S5 and S6)." However, the authors have been discussing dielectric anomalies around 300 K. Because the phase transition temperature is about 373 K, we have not yet seen repeatable peaks in permittivity at ferroelectric ordered temperatures. How to explain the variation of dielectric constant under cooling history and magnetic field dependence by proton mechanism? The author needs to provide a detailed explanation.
Reviewer #3 (Remarks to the Author): This work reports the ionic tuning of molecular ferroelectricity, thermal transfer and electronic memory, which is a new exciting frontier for molecular ferroelectrics. The proton transfer in molecular crystals (ferroelectrics in this work) triggers the fast diffusion-less mechanism, on which it induces the swift control of polarization and corresponding thermal conductivity through protonphonon coupling effect. The growth medium of molecular ferroelectrics and the humidity/water content provide a nice pathway towards the versatile tuning and control of ferroelectrics, only and especially for molecular ferroelectrics materials. The authors have carried out comprehensive studies from ferroelectric, thermal and electroresistive perspectives to uncover the novel stimuli tunable molecular ferroelectrics. The ionics using ion-controlled functional materials show a great potential to a conventional electronic platform, while ionics provide an extra ion tunability. One comment for the consideration is to introduce the criteria for ionic tuning (other than proton, any other potential candidates, etc.). Overall, this is a new research direction in molecular ferroelectrics with stimuli tuning ferroelectric/thermal/memory effects using ions. Therefore, I strongly recommend the publication of this article in Nature Communications.
Reviewer #4 (Remarks to the Author): The authors report stimuli responsive (proton and voltage) molecular ferroelectrics on which its ferroelectricity, thermal transport and electroresistance are tunable via proton ions. The structural, electrical and corresponding ferroelectric measurements provide a unified picture of the operation principles in such stimuli dependent molecular ferroelectrics, resulting from its molecular moieties, aqua environment and large lattice spacing. The ionic molecule ferroelectrics is a novel discovery in this report which could find a broad field potential applications after this (thermal transfer using ferroelectrics, memory, etc.). It would be interesting to provide some additional introductory description/discussion on thermal conductivity tuning using ferroics. Another comment is to provide its potential pathway utilizing thermal tunability.

     
The authors investigated molecular ionic ferroelectrics exhibiting the coexistence of room-temperature ionic conductivity (6.1×10-5 S/cm) and ferroelectricity, which triggers the ionic-coupled ferroelectric properties. Such ionic ferroelectrics with aqualigand molecules further present the controlled tunability in polarization from 0.68 to 1.39 C/cm2, thermal conductivity by 13 % and electrical resistivity by 86 % due to the proton transfer in an ionic lattice under external stimuli. In addition, the current version of this paper does not provide the insight result for the molecular ferroelectric ACTA PHYSICA SINICA 69 (21)s to guarantee the publication in Nature Communications. The following issue should be addressed before the publication. Reply:                     1) The same material ImClO4 were reported in the previous paper of these authors. The reference No. 30 is proton conductivity in the paper of Nat. Commun. 12, 255 4602 (2021). However, the same ImClO4 is also ionic ferroelectrics. Huang et al. reported the molecular ionic ferroelectric ImClO4, which exhibits coexistence of ionic conductivity and ferroelectricity. Interestingly, the coexistence of ionic conductivity and ferroelectricity can trigger the ionic-coupled ferroelectric properties, i.e., ionic controlled ferroelectric behavior. Because ferroelectric-ion coupling is of great significance in the development of the devices with multi-well polarized states and neuromorphic characteristics, the starting point of this work is interesting and worthy of affirmation. However, there are still many problems to be clarified or solved in the manuscript before publication of the work. The main problems are as follows: 1 The polarization value of the P-E curve in the manuscript is much lower than that of the same compound reported in the literature, and even lower than that of the same compound reported by the authors themselves (Sci. Adv. 2017, 3, e1701008) Figure 3, the author attributes the dielectric anomaly in a very wide temperature region near 300 K to the mobile proton released from the framework. However, this anomaly also exists in the heating stage of DSC, which is attributed to the structural phase transition.  Figures S5 and S6, the authors claim that "The repeatable permittivity peak at ferroelectric ordering temperature indicates high crystalline quality of as-grown ImClO4, further evidencing the proton mechanism for the anomaly in permittivity which relates to its cooling history and magnetic field dependence (Figs.S5 and S6)." However, the authors have been discussing dielectric anomalies around 300 K. Because the phase transition temperature is about 373 K, we have not yet seen repeatable peaks in permittivity at ferroelectric ordered temperatures. How to explain the variation of dielectric constant under cooling history and magnetic field dependence by proton mechanism? The author needs to provide a detailed explanation.